Many organizations, including businesses, governments and educational institutions, utilize computer networks in order to share and exchange information. A computer network typically comprises a plurality of entities interconnected by means of a communications media. An entity may consist of any device, such as a computer, that "sources" (i.e., transmits) or "sinks" (i.e., receives) data frames over the communications media. A common type of computer network is a local area network ("LAN") which typically refers to a privately owned network within a single building or campus. LANs typically employ a data communication protocol (LAN standard), such as Ethernet, FDDI or token ring, that defines the functions performed by data link and physical layers of a communications architecture (i.e., a protocol stack).
In many instances, several LANs may be interconnected by point-to-point links, microwave transceivers, satellite hook-ups, etc. to form a wide area network ("WAN") or subnet that may span an entire country or continent. One or more network devices are often used to couple LANs together and allow the corresponding entities to exchange information. For example, a network switch may be utilized to provide a "switching" function for transferring information, such as data packets, among entities of a computer network. Typically, the switch is a computer and includes a plurality of ports that couple the switch to the other entities. The switching function includes receiving data at a source port from an entity and transferring that data to at least one destination port for receipt by another entity.
Another network device is referred to as a router. A router is often used to interconnect LANs executing different LAN standards and/or to provide higher functionality than the switches, such as efficient message routing. To perform these tasks, a router, which is also often a computer, typically examines the destination address and source address of all packets passing through the router. Routers typically operate at the network layer of the protocol stack, such as the Transmission Control Protocol/Internet Protocol ("TCP/IP") reference model. If the LAN standards associated with the source entity and the destination entity are dissimilar (e.g., Ethernet and Token Ring), the router may alter the format of the packet so that it is in conformance with the standard corresponding to the destination entity.
An arrangement that is capable of associating any port of a switch with any particular segregated network group is disclosed in U.S. Pat. No. 5,394,402, issued on Feb. 28, 1995 to Floyd E. Ross (the "'402 Patent"). According to the '402 Patent, any number of physical ports of a particular switch may be associated with any number of groups within the switch by using a virtual local area network (VLAN) arrangement that virtually associates the port with a particular VLAN designation. Specifically, Ross discloses a switch or hub for a segmented virtual local area network with shared media access that associates VLAN designations with at least one internal port and further associates those VLAN designations with messages transmitted from any of the ports to which the VLAN designation has been assigned. The VLAN designation for each internal port is stored in a memory portion of the switch such that every time a message is received by the switch on an internal port the VLAN designation of that port is associated with the message. Association is accomplished by a flow processing element which looks up the VLAN designation in a memory based on the internal port where the message originated.
In many cases, it may be desirable to interconnect a plurality of these switches in order to extend the VLAN associations of ports in the network. Ross, in fact, states that an objective of his VLAN arrangement is to allow all ports and entities of the network having the same VLAN designation to interchange messages by associating a VLAN designation with each message. Thus, those entities having the same VLAN designation function as if they are all part of the same LAN. Message exchanges between parts of the network having different VLAN designations are specifically prevented in order to preserve the boundaries of each VLAN segment.
FIG. 1A illustrates a conventional computer network 100 comprising a plurality of LANs 101-115. The LANs 101-115, moreover, are interconnected by a plurality of switches 117-122. More specifically, each LAN 101-115 is preferably coupled to a specific internal port 116 of switches 117-122. The network 100 may also include a plurality of routers 123, 124, such that a router (e.g., router 123) is associated with a given segment of the network 100 (e.g., LANs 101-108). Switches 117-122 and routers 123, 124 are typically interconnected by trunk lines 128-133, which may be point-to-point links. Trunk lines 128-133 are typically coupled to specific external ports 125 of switches 117-122 and routers 123, 124.
Utilizing the system disclosed in the '402 Patent, various LANs may be grouped together to form VLANs. As shown in FIG. 1A, each VLAN may be identified by a color code, e.g., "R" for red, "B" for blue, "G" for green, "Y" for yellow and "O" for orange. More specifically, switches 117-122 associate each internal port 116 with at least one VLAN color tag. For example, switches 117 and 119 may each associate their ports coupled to LANs 102, 103 and 107 with the "red" VLAN designation, thereby grouping these LANs together. Since all entities located on a given LAN (e.g., LAN 102) utilize the same shared port 116 of the corresponding switch 117, moreover, each of these entities is also associated with the VLAN designation(s) assigned to that port (e.g., red).
Each external port 125 of switches 117-122 is similarly associated with one or more VLAN designation(s). The VLAN designations associated with external ports 125 correspond to the VLANs that are reachable through that external port 125. For example, switch 117 may associate its one external port 125 with the red VLAN designation, among others, to reflect that one or more LANs (e.g., LAN 107) associated with the red VLAN designation is "behind" that port.
To identify the various VLAN designations defined throughout the network 100, switches 117-122 and routers 123, 124 typically participate in a VLAN configuration protocol. In accordance with the protocol, each switch 117-122 periodically transmits information regarding the VLAN designation(s) assigned to its internal ports 116. Upon receipt, the other switches 117-122 typically utilize this information to assign VLAN designations to their external ports 125.
As set forth in the '402 Patent, any entity associated with the red VLAN designation may exchange messages with any other similarly designated entity, even though the two entities may be physically remote from each other and associated with switches that are coupled to other VLANs. For example, assume that entity 17 which is coupled to a "red" internal port 116 at switch 117 wishes to send a message (i.e., a unicast message) to entity 18 coupled to a "red" port at switch 119. As shown in FIG. 1B, entity 17 generates a message 150, which typically includes a start flag 151, a destination address 152 (corresponding to entity 18), a source address 154 (corresponding to entity 17), information 156 comprising the message and an end of message flag 158. Entity 17 then drives message 150 on the corresponding LAN 103.
Switch 117 receives the message on an internal port 116 associated with the red VLAN and, as shown in FIG. 1C, attaches a red VLAN designation field 160 to the message 150. Although the VLAN designation 160 is shown appended to the message 150, those skilled in the art will understand that it may be attached at any point within the message. Switch 117 also examines the destination address (entity 18) and determines that network entity 18 is not coupled to an internal port 116 of switch 117, but that its one external port 125 is associated with the red VLAN designation, among others. Accordingly, switch 117 drives message 150 with VLAN designation 160 onto port 125. Switch 117 may additionally encapsulate message 150 for transmission over trunk 128.
Message 150 is next received at switch 118. Since the only port on switch 118 associated with the red VLAN designation (other than the port on which the message arrived) is its second external port 125 (corresponding to trunk 129), the message 150 is switched to that port for transmission to switch 119. Upon receiving the message, switch 119 examines the VLAN designation 160 and destination address 152 and determines that it corresponds to entity 18 which resides on LAN 107. Switch 119 further determines that "red" VLAN designation of the message 150 matches the designation associated with the internal port 116 coupled to LAN 107. Accordingly, switch 119 strips off the VLAN designation field 160 and drives message 150 onto internal port 116 coupled to LAN 107.
To improve the flexibility of network 100, it is desirable to support the transmission of message to a predefined group of entities, including entities of diverse VLAN designations. Such a message is known as a group multicast message. For example, a group of entities (e.g., entities 18-25) may wish to receive certain types of messages originating from entity 17. These entities typically register with one or more multicast routers (e.g., router 123) which controls the transmission of messages across VLAN boundaries. Router 123 may employ a security mechanism to prevent entities having a particular VLAN designation from subscribing to multicast messages to which they not entitled.
Registration may be accomplished via the Internet Group Management Protocol (IGMP) which defines operations that may be used by entities to join a group (e.g., JoinGroup). To limit the amount of traffic, only one entity per LAN typically transmits such a request. Other interested entities will rely on the first subscription request to cause the message to be delivered to the LAN, thereby making it available to all interested entities. The switches may perform additional filtering such that router 123 receives only one subscription request per VLAN. In response, router 123 creates a subscription list (not shown) associating each group multicast address with the list of VLANs on which at least one subscribing entity resides. Thereafter, entity 17 utilizes this group destination address to send multicast messages to the subscribing entities.
Switches 117-122 may also monitor the IGMP messaging between entities and multicast router 123 to learn which ports 116, 125 lead either to a router or to at least one entity subscribing to a particular group multicast address. As discussed above, however, switches 117-122 may not exchange messages between segments of the network 110 associated with different VLAN designations (e.g., between entities associated with the red and blue VLANs). To support cross-VLAN traffic, switches 117-122 deliver all multicast messages to the multicast router 123.
Entity 17 generates a multicast message by entering the predefined group multicast address in the destination address field 152 (FIG. 1B). The multicast message is then driven on LAN 103 and received by switch 117. As with unicast messages, switch 117 attaches the corresponding VLAN designation 160 associated with the port 116 on which the message was received. Switch 117 also determines that the message is a group multicast message and (assuming the switch participates in IGMP) accesses its subscription list to determine which internal and external ports 116, 125 are associated with this address. Since only LAN 103 and trunk 128 are associated with this group multicast address and the message was received from LAN 103, switch 117 drives the message onto trunk 128 via external port 125 where it is received by switch 118.
Switch 118 examines the destination address of the message and its own subscription list which indicates that LAN 106 and trunks 128 and 129 are associated with this group multicast address. The "red" VLAN designation of the message, however, does not match the "blue" VLAN designation associated with the internal port 116 coupled to LAN 106 and thus switch 118 is precluded from forwarding the message onto this internal port 116. Furthermore, since the message was received on trunk 128, switch 118 simply drives the message onto its external port 125 coupled to trunk 129.
Switch 119 receives the message, examines the VLAN designation and destination address and determines that LAN 107 and trunks 129 (on which the message was received), 130 and 131 are associated with this group multicast address. Here, the red VLAN designation of the message matches the VLAN designations associated with port 116 coupled to LAN 107 and with port 125 coupled to trunk 130, but not with port 125 coupled to trunk 131. Accordingly, switch 119 strips off the VLAN designation field 160 and forwards the message onto internal port 116 for transmission over LAN 107. Switch 119 also drives the message with the VLAN designation field 160 on its external port 125 coupled to trunk 130 for receipt by multicast router 123.
Multicast router 123 examines the message and determines that it is a "red" multicast message from entity 17. Router 123 accesses its subscription list and determines this group multicast address is associated with the red, blue, green, yellow and orange VLANs. Since the VLAN designation of the message matches one of the associated VLANs (i.e., red), router 123 presumes that message has already been delivered to the subscribers on that VLAN (e.g., entity 19). That is, switches 117-122, as described above, are able to deliver multicast messages in which the VLAN designation of the message matches the VLAN designation of the subscribing entities. Thus, multicast router 123 need not forward multicast messages to the same VLAN designation as the sourcing entity. Because switches 117-122 were precluded from forwarding the multicast message across non-matching VLANs, router 123 proceeds to forward the message to those VLANs (i.e., the blue, green, orange and yellow VLANs).
To deliver multicast messages to subscribers of diverse VLAN designations, router 123 must create a separate message for each of the remaining VLAN designations. That is, router 123 creates four identical copies of the message, each with a different VLAN designation attached thereto (i.e., blue, green, yellow and orange). Each copy is then sent out on the network 100 by router 123.
As these messages are received by the switches 118-122, they are directed to internal ports associated with the corresponding blue, green, yellow or orange VLAN designations to which a subscribing entity is coupled. That is, the first copy having the blue VLAN designation is received and sent out by switch 118 on the "blue" port 116 coupled to LAN 106 (for receipt by entities 19 and 20). The second copy is forwarded by switch 119 on its "green" port 116 coupled to LAN 108 for receipt by entity 21. Similarly, the third copy of the message is delivered to entities 22 and 23 associated with the yellow VLAN at switch 120 and the fourth copy to entities 24 and 25 associated with the orange VLAN at switches 120 and 122. In this manner, the message is transmitted to all LANs on which at least one subscribing entity resides.
Although the prior art arrangement as described above is capable of delivering multicast messages to entities of diverse VLAN designations, it nonetheless has certain disadvantages. First, the arrangement requires that the same message be copied multiple times by the multicast router; one copy for each VLAN associated with the message, In addition, each copy must be placed on the trunk line(s) linking the multicast router to the computer network. Depending on the number of VLAN designations associated with a given group multicast address, this may severely compromise the throughput on this trunk line. Multicast messaging may thus subject the network to substantial performance penalties, limiting the benefits of having established virtual local area networks.